The cytochrome P450s comprise the principal monooxygenase system which metabolizes foreign chemicals. There are genetic polymorphisms in many of these enzymes including the CYP2C subfamily (S-mephenytoin metabolism) in man. We have isolated and characterized cDNAs for all of the known members of the CYP2C subfamily including cDNAs for two new genes (2C18 and 2C19). S-mephenytoin metabolism in human liver correlated highly with 2C18 mRNA (r = 0.95) and to a lesser extent with 2C9 (r = 0.65), suggesting that one or both of these isozymes are involved in this polymorphism. Six members of the 2C subfamily (2C8, 2C9(2 alleles), 2C18(2 alleles), and 2C19)) were expressed in a yeast cDNA expression system and the recombinant P450s purified from yeast. 2C18 stereospecifically 4'-hydroxylated S- mephenytoin, while neither 2C8 nor 2C9 metabolized this substrate. In contrast, the Ile359 allele of 2C9 metabolized tolbutamide preferentially, while the substitution of one amino acid (Leu359) essentially abolished activity. These results suggest that 2C18 is important in S-mephenytoin metabolism and 2C9 in tolbutamide metabolism, but that allelic variations may also affect metabolism. The anti-coagulant warfarin was also region and stereospecifically metabolized by this subfamily. 2C9 7-hydroxylated S-warfarin, while 2C18 4-hydroxylated S warfarin. The 2C cDNAs have been inserted into the AHH/TK- cell line, and future studies will characterize their ability to metabolize premutagens. Site-specific mutagenesis was used to produce know allelic variations of human 2C9. These have been tested for their effects on warfarin metabolism and will be tested for their ability to alter metabolism of S-mephenytoin and tolbutamide. The contribution of these enzymes to benzpyrene metabolism will be addressed in future studies. Upstream regions of 2C18 and 2C9 have been isolated. Future studies will address the role of allelic variations in coding and regulatory regions on the ability of humans to metabolize selected drugs. Other studies are addressing the variability of human 1A1 and 1A2 in human liver.